debian-installer/packages/partman-efi/check.d/efi starts with these lines:
if [ ! -d /proc/efi ] && [ ! -d /sys/firmware/efi ]; then
exit 0
fi
if [ -f /var/lib/partman/ignore_uefi ]; then
exit 0
fi
So it seems installation for EFI is only supported if the installer is
booted via EFI.
The armhf netinstall iso does not make use of the UEFI implementation of
u-boot in contrast to what Suse and Fedora do. So we get stuck in a
legacy mode of booting. This is unsatisfactory if we want to setup a
multi-boot installation.
Best regards
Heinrich